This internal flag has been hardcoded to True for several years and
we do not utilize the "False" case anywhere, nor do we mention it in
the docs or expose it to the end users.

The leaser thread should not terminate implicitly when the streaming
pull manager's consumer thread becomes inactive, as there might still
be messages being processed inside the scheduler and we want to keep
extending these messages' ACK deadlines until the scheduler has been
shut down.

NOTE: The manager's streaming pull RPC does not need to be active, since
all mod-ACK requests are sent using a separate unary request instead of
over the stream.

The library only supports Python 3.6+, thus we don't need the
conditional anymore.

For consistency with the leaser, the heartbeater should not terminate
implicitly when the manager stops the background stream, but should
instead wait for the manager to stop it explicitly.

After the streaming pull manager shuts down the consumer thread and
becomes "inactive", there might still be requests waiting in the queue
to be dispatched, thus the dispatcher should not implicitly enter the
no-op mode of operation.